Question: Solve for $a$ : $8 + a = 15$
Subtract $8$ from both sides: $ \begin{eqnarray} \\ 8 + a &=& 15 \\ \\ {-8} && {-8} \end{eqnarray} $ $ \begin{eqnarray} \hphantom{8 + a} &\hphantom{=}& \hphantom{15} \\ a &=& 15 {- 8} \\ \end{eqnarray} $ Simplify. $ a = 7$